There is a current demand to manufacture as compact as possible disk drives and other devices for driving data storage media, and accordingly, there is a corresponding demand to manufacture as compact as possible the motors used to drive the media in such devices. In some applications, it is also desirable that such motors be as thin as possible.
Conventional motors used for driving storage media, such as floppy disks, typically employ a pair of ball bearing assemblies aligned with each other in the direction of the rotational axis of the motor to ensure stable operation of the motor. In order to reduce the height or thickness of a motor, i.e., the dimension of the motor in its axial direction, certain prior art motors have employed only a single ball bearing assembly. However, in these types of motors, additional measures have been required to ensure stable operation. Otherwise, because only a single ball bearing is employed, the motor would vibrate, wobble and/or wave the media during rotation, and in turn cause errors in recording and reproducing data on the media.
Japanese laid-open patent application number Hei6-202186 (Publication No. Hei 8-63872) shows a prior art approach for stabilizing the rotation of a motor, wherein the magnetic centers of the stator and driving magnet of the motor are offset relative to each other in the axial direction of the motor. This offset applies a magnetic bias to the ball bearing which is intended to eliminate or reduce any looseness in the bearing and, in turn, stabilize rotation of the motor. One problem associated with this approach in relatively thin motors is that the axial thicknesses of the stator and driving magnet are relatively small, and therefore the magnetic bias applied to the ball bearing is insufficient to stabilize rotation of the motor. The motor of this Japanese laid-open patent application employs an oil-impregnated bearing in addition to the single ball bearing. The oil-impregnated bearing is formed of a ring-like metal member which is impregnated with oil and seated between the rotatable shaft and a stationary member of the motor
Another thin type motor having a thickness of several millimeters and employing an oil-impregnated bearing is illustrated in FIG. 1. The motor of FIG. 1 is essentially the same as that illustrated in FIG. 6 of co-pending U.S. patent application Ser. No. 09/114,007, which is assigned to the Assignee of the present invention, and is hereby expressly incorporated by reference as part of the present disclosure. The motor of FIG. 1 includes a cover plate 2 extending over an opening formed in a stationary member 1, such as a chassis of a device for driving a floppy disk. A rotor hub 6 is formed integral with an upper end portion of a rotatably-mounted shaft 3. A rotor yoke 8 is formed of a magnetic material, such as a ferrous material, and is fixedly fitted by means of coupling member 7 around a peripheral portion of the rotor hub 6.
The rotor yoke 8 is composed of a substantially plate-like base 8a and a depending or bent-down section 8b. The base 8a defines an opening 9, and the bent-down section 8b is formed integral with the base and projects downwardly from a peripheral end portion of the base. The coupling member 7 is fitted within the opening 9 of the base 8a. A chucking magnet 10 is attached to an upper face of the base 8a around the periphery of the opening 9. The chucking magnet 10 serves to attract a metal portion 11 on the backside of a floppy disk, as shown in broken lines in FIG. 1, to secure the floppy disk on the rotor.
As also shown in FIG. 1, a primary drive magnet 15 is fitted within the bent-down section 8b of the rotor yoke 8 via an insertion member 14. A stator 16 is arranged under the rotor yoke 8 so as to oppose the main magnet 15 with a predetermined space or gap therebetween. The -stator 16 is formed with a plurality of teeth 16a of a core (only one tooth is shown in the Figure), and an electromagnetic coil 16b is wound around each of the teeth.
A ring-like, oil-impregnated bearing 18 is fitted around a lower peripheral portion of the shaft 3, and a ring-shaped bearing holder 19 is fixedly coupled or connected with the stationary member 1 and arranged outside of the oil-impregnated bearing. Also, an opening 20 is formed through the center of the bearing holder 19, and a circular projection or upright section 21 extends around the periphery of the opening. The oil-impregnated bearing 18 is fixedly fitted within the opening 20 of the circular projection 21 so that an outer peripheral face of the oil-impregnated bearing 18 slides around an inner peripheral face of the circular projection 21.
As further shown in FIG. 1, an elevated section 22 is formed on a radially outward upper peripheral surface of the bearing holder 19. The stator 16 is supported in such a manner that an inner peripheral portion of the core of the stator is fitted to the elevated section 22. An inner race 24a of a ball bearing 24 is fixedly fitted to the outside of the circular projection 21 of the bearing holder 19 to thereby support the ball bearing on the bearing holder. An outer race 24b of the ball bearing 24, on the other hand, is fixedly fitted onto an inner peripheral wall 7a of the coupling member 7.
In this type of construction, the shaft 3 rotates with the rotor hub 6, the coupling member 7, the rotor yoke 8, the oil-impregnated bearing 18, and the outer race 24b. The inner race 24a of the ball bearing 24, on the other hand, is not rotated, but rather is fixedly secured to the bearing holder 19. Thus, the outer peripheral surface of the oil-impregnated bearing 18 slides around the inner peripheral surface of the circular projection 21 of the bearing holder 19.
The relatively thin motor shown in FIG. 1 is an outer rotor type motor wherein the rotor extends radially outward beyond the stator 16, and the primary drive magnet 15 is located outside of the stator. The rotor has a radius, i.e. the distance from the center of rotation to the outer periphery of the rotor, which is more than twice as large as the thickness of the motor in the axial direction. As described above, the thickness of the motor is reduced by the use of a single ball bearing 24. In addition to the single ball bearing 24, the oil-impregnated bearing 18 is provided to reduce vibration of the rotor, which is composed of the rotor hub 6 and the rotor yoke 8, during its rotation.
One drawback associated with this type of bearing structure arises from the bearing holder 19 being mounted between the oil-impregnated bearing 18 and the ball bearing 24. When the finishing accuracy on the inner and/or outer peripheral surfaces of the circular projection 21 is inadequate, and particularly when the finishing accuracy of the portions of these surfaces which contact the oil-impregnated bearing 18 and/or the ball bearing 24 is inadequate, rotational irregularity may be caused in the motor which can, in turn, significantly affect or reduce the rotational characteristics of the motor.
In addition, relatively complicated processing must be applied to the bearing holder 19 in order to manufacture the bearing holder as shown. As a result, a relatively hard material cannot be used to form the bearing holder. These factors further enhance the difficulties associated with increasing the life of the oil-impregnated bearing 18.
Another drawback associated with bearing structures of this type, is that the bearing holder 19 mounted between the oil-impregnated bearing 18 and the bearing 24 makes it difficult to reduce the dimension of the motor in its radial direction.
Accordingly, a primary object of the present invention is to provide a bearing structure which allows the motor to be both relatively thin in its axial direction and to achieve stable operation.
It is another object of the present invention to provide a thin type motor which is substantially free from irregular rotation.
It is still another object of the present invention to provide a motor for driving data storage media which allows data to be accurately written onto or read from the media. It is a further object of the present invention to provide a motor for driving data storage media, such as a thin motor of the type described, wherein the preload or magnetic bias applied to the ball bearing is sufficient to achieve stable rotation of the motor, and thereby avoid errors in writing data to or reading data from a floppy disk or like storage medium that otherwise might be caused from rotational instability.
A still further object of the present invention is to improve the rotational characteristics of thin type motors, and if oil-impregnated bearings are used in cooperation with single ball bearings to improve the rotational stability of such motors, to increase the useful lives of such oil-impregnated bearings.